In his latest RPG Scrollbars column for Rock, Paper, Shotgun, Richard Cobbett discusses the various games that attempted to implement a Dungeon Master mode throughout the years, and why ultimately they all failed to provide an experience that feels even remotely comparable to a tabletop game session. Here's an excerpt:

If the DM has one job here, it's to try and disguise it with hand-crafted content and a human eye for the rules and systems. The catch is that even using something as powerful as the Neverwinter Nights editor to create a complex module full of wonder and whimsy, once the game starts that player just becomes another puppet of the game engine albeit one with the power to see and occasionally twang everyone else's strings. They're not in charge of the rules, because that's the game logic, and even the simplest of engines makes it a pain to create content on the fly. Sure, you can make a module or a map in advance, but then you're still stuck with the problem of not being able to react with the speed of thought to game limitations and player freedom. Which as ever, will usually manifest in variously murderous killing sprees.

What does it usually boil down to letting you do? Add monsters on the fly and twiddling. That's about the extent of what a modern game can allow, with the result that what should be the mode that expands a game and makes it seems full of possibilities actually ends up just revealing the limitations all the stronger how the traps work for instance, how progression is kept out of the GM's hands with loot tables and equipment controlled by someone else, how it's not possible to have a secret door unless someone has specifically coded that in advance. It's creation within a straitjacket, further tied down by interface design and balance. You're not working hand in hand with the source material to create something great, you're working for it. You're not the master of the game, you're its unpaid middle-management.